3. Spectroscopy

Question 1

What is the Beer-Lambert law?

Answer 1

this relates to the absorbance (A) of a solution to the molar absorbance coefficient (ξ) of the absorbing species, the concentration © of the absorbing species and the path length of the cell (l), via the equation A = ξ c L

Question 2

What is bending?

Answer 2

a type of vibration that changes bond angles

Question 3

What is Chemical shift (δ)?

Answer 3

the quantity used in NMR spectroscopy to identify the positions of signals produced by the nuclei of a sample. The unit of chemical shift (δ) is parts per million (ppm)

Question 4

What is 13C-NMR spectroscopy?

Answer 4

Nuclear magnetic resonance spectroscopy of carbon atoms within molecules. It provides information about the carbon-hydrogen framework of a molecule

Question 5

What is a complex splitting pattern?

Answer 5

a splitting pattern resulting from more than one set of equivalent hydrogen atoms. Signals of this type are called multiplets

Question 6

What is a correlation table?

Answer 6

a table of data on spectroscopic absorption patterns of selected functional groups

Question 7

What does deshielded mean?

Answer 7

describes the situation in NMR spectroscopy in which resonance or inductive effects reduce the electron density around a nucleus, thus increasing the ability of an applied magnetic field to bring the nucleus into resonance

Question 8

What is a doublet?

Answer 8

a signal in 1H-NMR spectroscopy that has been split into two peaks in a ratio of 1:1

Question 9

What does downfield mean?

Answer 9

Describe a signal in NMR spectroscopy that is towards the left of the spectrum or of another signal

Question 10

What is electromagnetic radiation?

Answer 10

energy propagated through space in the form of oscillating electric and magnetic fields

Question 11

What is electron spin resonance (ESR)?

Answer 11

a methods of locating electrons within the molecules of a paramagnetic substance

Question 12

What are equivalent hydrogen atoms?

Answer 12

hydrogen atoms that are chemically equivalent

Question 13

What is the fingerprint region?

Answer 13

in infrared spectroscopy, the region of the spectrum from 1000 to 400 cm-1

Question 14

What is frequency?

Answer 14

the number of full cycles of a wave that pass a given point in a second

Question 15

What is Hertz (Hz)?

Answer 15

the SI unit of frequency. One hertz equals one wave cycle per second

Question 16

What is high resonance mass specroscopy?

Answer 16

a technique that allows the precise measurement of the mas to charge ratio of ions

Question 17

What is 1H-NMR spectroscopy?

Answer 17

nuclear magnetic resonance spectroscopy of hydrogen atoms within molecules. It provides information about the carbon-hydrogen framework of a molecule

Question 18

What is the index of hydrogen deficiency?

Answer 18

the sum of the number of rings and π-bonds in a molecule

Question 19

What does infrared active mean?

Answer 19

any vibration that results in the absorption of infrared radiation. For a molecule to absorb infrared radiation, the bond undergoing vibration must be polar and its vibration must cause a periodic change in the bond dipole; the greater the polarity of the bond, the more intense the absorption

Question 20

What is infrared radiation?

Answer 20

the portion of the electromagnetic spectrum with wavelengths in the range 7.8 x 10-7 m to 2.0 x 10-3m

Question 21

What is integration?

Answer 21

a mathematical process used for determining the area under a signal in an NMR spectrum

Question 22

What are lines of integration?

Answer 22

a curved line at a signal on a 1H-NMR spectrum that allows the determination of the relative number of hydrogen atoms giving rise to that signal

Question 23

What is a local magnetic field?

Answer 23

the magnetic field generated by electrons surrounding a nucleus

Question 24

What is mass spectroscopy?

Answer 24

a technique used to determine the mass of molecules and fragments of molecules

Question 25

What is a molecular ion?

Answer 25

an ionised molecule that produces a peak in a mass spectrum. This is usually the peak with the highest m/z value

Question 26

What is a mulitplet?

Answer 26

the splitting pattern of a complex signal in 1H-NMR spectroscopy

Question 27

What is multiplicity?

Answer 27

the splitting pattern of a signal in 1H-NMR spectroscopy, which can be described as, singlet, doublet, triplet, quartet, multiplet etc.

Question 28

What is the (n + 1) rule?

Answer 28

a rule for determining the splitting patterns in NMR spectroscopy. If hydrogen atom has n other hydrogen atoms that are not equivalent to it, but are equivalent to each other, on the same or adjacent atom(s), its 1H-NMR signal is split into (n+1) peaks

Question 29

What is nuclear magnetic resonance (NMR) spectroscopy?

Answer 29

a spectroscopic technique that measures the absorption of energy by nuclei in the presence of a magnetic field

Question 30

What is a quartet?

Answer 30

a signal in 1H-NMR spectroscopy that has been split into four peaks in a ratio of 1:3:3:1

Question 31

What is a radical cation?

Answer 31

a species with a positive charge and an unpaired electron

Question 32

What is radio-frequency radiation?

Answer 32

the portion of the electromagnetic spectrum with wavelengths greater than a metre

Question 33

What is a reference compound?

Answer 33

a compound added to a sample to be studied by NMR spectroscopy, usually TMS. The positions of the signals in the spectrum are then compared with those of the reference compound

Question 34

What is resonance?

Answer 34

the absorption of electromagnetic radiation by a spinning nucleus and the resulting flip of its nuclear spin state

Question 35

What does shielded mean?

Answer 35

describes the situation in NMR spectroscopy in which local magnetic fields from electron surrounding a nucleus decrease the ability of an applied magnetic field to bring the nucleus into resonance

Question 36

What is signal splitting?

Answer 36

a phenomenon in NMR spectroscopy in which the 1H-NMR signal from one set of hydrogen atoms is split by the influence of neighbouring non-equivalent hydrogen atoms

Question 37

What is a singlet?

Answer 37

a signal in 1H-NMR that has not been split

Question 38

What is a splitting pattern?

Answer 38

the pattern obtained when a signal in a 1H-NMR spectrum is split by the influence of neighbouring non-equivalent hydrogen atoms. These can be described as multiplets, singlets, doublets, triplets, quartets etc.

Question 39

What is stretching?

Answer 39

a type of vibration that changes bond lengths

Question 40

What is tetramethylsilane (TMS)?

Answer 40

the reference standard used in NMR spectroscopy. Its signal is set at δ = 0

Question 41

What is a triplet?

Answer 41

a signal in 1H-NMR spectroscopy that has been split into three peaks in a ratio of 1:2:1

Question 42

What does upfield mean?

Answer 42

describes a signal in NMR spectroscopy that is towards the right of the spectrum or of another signal

Question 43

What is UV/visible spectroscopy?

Answer 43

a technique used to study compounds that absorb light in the ultraviolet-visible region

Question 44

What is vibrational infrared?

Answer 44

the portion of the infrared region with a frequency range of 400-4000cm-1

Question 45

What is wavelength (λ)?

Answer 45

the distance between any two consecutive identical points on a wave

Question 46

What is wavenumber?

Answer 46

the number of wavelengths per centimetre. The unit of wavenumber is the reciprocal centre metre (cm-1) and is commonly used infrared spectroscopy

Question 47

What is X-ray crystallography?

Answer 47

the study of crystal structures by X-ray diffraction techniques

Question 48

What spectroscopic technique is used to determine molecular weight of a compound?

Answer 48

Mass spectroscopy

Question 49

How many decimal places does High-resolution mass spectroscopy give an accurate reading to?

Answer 49

Five decimal placed

Question 50

What spectroscopic technique is used to detect functional groups?

Answer 50

Infrared (IR) spectroscopy

Question 51

What determines whether a bond is detectable via IR spectroscopy?

Answer 51

It must have a dipole moment

Question 52

What is the most useful spectroscopic technique?

Answer 52

Nuclear Magnetic Resonance (NMR) Spectroscopy

Question 53

What does NMR detect?

Answer 53

Functional groups Number of different groups Connectivity of different groups

Question 54

How do you determine the number of hydrogens in a particular environment?

Answer 54

The area under the peak of the NMR graph

Question 55

What is the units used in NMR?

Answer 55

Parts per million (not a concentration)

Question 56

What is the chemical shift of Tetramethylsilane (TMS)?

Answer 56

0

Question 57

True or false: If a molecule has a plane of symmetry it simplifies the NMR spectrum and we only need to examine one half to determine the NMR spectrum

Answer 57

True

Question 58

Do chemically equivalent protons show splitting?

Answer 58

No

Question 59

Why does splitting occur in NMR?

Answer 59

The protons can be influenced by the magnetic fields of the neighbouring hydrogens.

Question 60

What is the formula for determining the number of protons per hydrogen environment based on NMR data?

Answer 60

n + 1 peaks (n = equivalent hydrogens)

Question 61

Where would a band be seen for an alcohol undergoing IR spec?

Answer 61

Broad peak around 3000-3500cm-1

Question 62

Where would a band be seen for a molecule with a carbonyl group undergoing IR spec?

Answer 62

Strong sharp band around 1700cm-1

Question 63

What is the IHD formula?

Answer 63

0.5 x [2c+2-h-x+n]